The invention relates to a pressure swing adsorption process for the purification and separation of a gaseous mixture, and in particular to a sequential system wherein certain components, e.g. H.sub.2 O are removed by adsorption in a first adsorption bed and certain other components are removed in a downstream second adsorption bed.
For the past several years, pressure swing gas adsorption processes have proven to be practicable for many purposes. In these processes, the adsorption occurs at a higher pressure than the desorption of the previously adsorbed components, which desorption is simply effected by a pressure drop, optionally until subatmospheric pressure and optionally by the additional use of a purge gas. The changes in pressure occur in a period of time measuring only minutes or seconds. A further, essential feature of the known pressure swing processes is the fact that neither heating at desorption nor cooling at adsorption is required. Consequently, since these processes take place essentially at a constant temperature, preferably at room temperature, they are extraordinarily favorable in terms of energy. In the conventional pressure swing methods, an adsorption zone of the preferably adsorbed components is produced within the adsorbent bed, and the front of this zone travels towards the outlet of the adsorber during the adsorption step and is displaced in the opposite direction during desorption, so that the front oscillates back and forth, in both steps. This adsorption front is called the "charging front" or "adsorption wave."
A conventional process of this type, the details being incorporated by reference herein, is described in DOS [German Unexamined Laid-Open Application] No. 2,055,425, and British patent specification No. 1,375,728.
If zeolites are employed for the separation of a gas mixture, then steps must be taken to ensure that the gas mixture to be separated enters the adsorbers largely predried, i.e., pre-purified; otherwise, the zeolites, due to their lattice structure, will selectively adsorb water over all other substances, resulting in the substantial if not total reduction in the adsorption capacity of the zeolites for the other components. In the conventional process which relates to the production of a fraction enriched with oxygen compared to the air, distinct and separate adsorbent-type dryers are therefore connected upstream of the product adsorbers. These dryers are filled with a water-selective adsorbent, e.g., silica gel or zeolite.
Since the dryer-adsorber pair is always connected by a conduit, the buildup of the adsorption pressure in both containers creates a special problem in the conventional process. Therefore, in the conventional process a throttle point is installed between the two containers, which throttle point permits the increase in pressure in the product adsorber to be slowed down during the buildup with crude gas which takes place from the inlet end of the dryer.
Aside from the special throttle point, the conventional process displays some significant disadvantages. Firstly, for the process to be conducted, there is always the requirement for two separate containers connected by a conduit for the drying and adsorption means. Secondly, the gas needed for the re-compression of the adsorber must be introduced by way of the inlet to the dryer. In that way the water charging front is displaced in the dryer in the direction of the outlet end of said dryer, and this means that after a prolonged operation, the water charging front does approach the outlet end of the dryer, and the water front may break through and penetrate the main adsorber. To guard against this possibility, the dryer must be substantially oversized.
A pressure buildup with a dry and CO.sub.2 -free gas, like the one produced, for example, in the conventional process from the cocurrent expansion of an adsorber, can only be carried out by passing said gas into the outlet end of the adsorber which in turn leads to an undesired contamination of the outlet layer of zeolite with unadsorbed low boiling impurities, e.g., nitrogen present in the dry and CO.sub.2 -free gas. Conversely, if such a gas is introduced by way of the dryer, the danger exists that water from the dryer is desorbed by this gas and carried into the zeolite layer. Thus, if a product of high purity is to be produced with the conventional process, it is necessary to forego the utilization of this gas, thereby resulting in poorer yields of product gas.